Gypsum wallboard is manufactured by preparing slurry of calcined gypsum and other additives with an excess of water, forming this slurry into board form within an envelope of wallboard paper, and allowing the gypsum to harden while supported in board form. A great excess of water must then be removed, in a high temperature dryer, this moisture coming out of the gypsum core by passing through the pores of the paper. In order for the moisture to be able to pass through the paper at an acceptable rate, in present day high speed plants, it is essential that the paper has a porosity value of about 300 seconds or lower, as determined on a Gurley densometer, using TAPPI Standards T460 M-49 test procedure.
It has long been recognized that it can be advantageous to improve the properties of the paper surface of a gypsum wallboard, but that treatment of the paper surface before formation into a board generally causes manufacturing problems.
Paper for gypsum board is conventionally made by pulping up waste paper constituents of old corrugated paper, or kraft cuttings and waste news. In cleaning, screening and refining the suspended materials in water suspension, the processed paper stock is diluted still further with water and then formed by draining the plies of paper on several continuously moving wire cylinders, where the separate plies are joined together by a carrying felt. The weak paper web is then dewatered in a press section where water is pressed out of the web. The pressed paper is dried in a multi-cylinder drying section with steam added to each cylinder. The dried paper is subjected to a squeezing or calendaring operation for uniformity in thickness and is then finally wound into rolls. The paper is subsequently utilized as paper cover sheets to form gypsum wallboard by depositing a calcined gypsum slurry between two sheets, and permitting the gypsum to set and dry. More complete details concerning the processing of paper for use in gypsum wallboard can be found in U.S. Pat. No. 4,959,272 of Long, the disclosure of which is incorporated herein by reference in its entirety.
U.S. Pat. No. 5,772,847 to Simpson et al. discloses a method for forming pulp from processed recycled fibers which includes the treatment of processed recycled fibers in a pulper with an aqueous composition that includes a caustic agent, such as sodium hydroxide, a buff/dispersing agent, a bleaching agent, a chelating agent, and a de-inking agent before the pulp is processed into a paper product. The reference does not address the use of the paper for making gypsum wallboard or the unexpected improvement in the nail pull strength of the wallboard made with the processed fibers.
U.S. Pat. No. 5,503,709 to Burton discloses an environmentally improved process for preparing recycled ligno-cellulosic materials for bleaching that includes the use of caustic for treatment of repulped material. The properties of the repulped paper or its improvement in the nail pull strength of gypsum wallboard made with the repulped material is not disclosed.
U.S. Pat. No. 6,991,705 to Leino et al. discloses a process for stabilizing the pH of a pulp suspension with both sodium hydroxide and carbon dioxide and for producing paper from the stabilized pulp, but does not teach the use of the paper in gypsum wallboard or the improvement of the nail pull strength of gypsum wallboard made with the pH stabilized pulp
U.S. Pat. No. Re. 36,424 to Clement discloses a method for producing pulp from printed “unselected” waste paper that uses alkaline treatment of the waste paper to remove ink.